Clinical characteristics of 4,520 paediatric patients infected with the SARS-CoV-2 omicron variant, in Xi'an, China

Background and objective Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has broad tissue tropism and high transmission, which are likely to perpetuate the pandemic. The study aim to analyze the clinicopathogenic characteristics in paediatric patients. Methods In this single-centre study, we retrospectively included all confirmed cases infected by SARS-CoV-2 infection at Xi’an Children's Hospital, China, from 1 December to 31 December 2022. The demographic, clinical, laboratory, and radiological features of the patients were analysed. Results A total of 4,520 paediatric patients with SARS-CoV-2 omicron variant infections were included. Of these, 3,861 (85.36%) were outpatients, 659 (14.64%) were hospitalised patients, and nine patients (0.20%) died. Of the nine patients who died, five were diagnosed with acute necrotising encephalopathy (ANE). The most common symptoms were fever in 4,275 (94.59%) patients, cough in 1,320 (29.20%) patients, convulsions in 610 (13.50%) patients, vomiting in 410 (9.07%) patients, runny nose/coryza in 277 (6.13%) patients, hoarseness of voice in 273 (6.04%) patients. A blood cell analysis showed a slight elevation of monocytes (mean: 11.14 ± 0.07%). The main diagnoses for both outpatients and inpatients were respiratory infection with multisystem manifestations. Conclusions A high incidence of convulsions is a typical characteristic of children infected with SARS-CoV-2. Five of the nine COVID-19 fatalities were associated with ANE. This indicates that nervous system damage in children with SARS-CoV-2 infection is more significant.


Introduction
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a permanently pandemic of acute respiratory disease (1,2)."Omicron", a new variant of the Coronavirus (B.1.1.529)has led to a global pandemic (3).These new BQ and XBB sub-variants of Omicron are transmitted far more rapidly than other variants of SARS-CoV-2 (4)(5)(6).SARS-CoV-2 is highly transmissible with broad tissue tropism that is likely perpetuating the pandemic (7).The infection of humans with SARS-CoV-2, lead to severe "flu"-like symptoms that can progress to pneumonia, acute respiratory distress, renal failure, and death (8)(9)(10).This study aimed to investigate the clinical characteristics of a large sample of paediatric patients infected with the omicron variant who were diagnosed and treated at our hospital.
2 Materials and methods

Patient selection
This retrospective, observational, single-centre study was conducted from 1 December to 31 December 2022.A total of 4,520 paediatric patients were infected with the omicron variant at Xi'an Children's Hospital (The Affiliated Children's Hospital of Xi'an Jiaotong University) in Xi'an, China.All patients enrolled in this study were diagnosed according to World Health Organization interim guidance (11).Throatswab specimens from the upper respiratory tract that were obtained from all patients at admission were maintained in viraltransport medium.2019nCoV was confrmed by realtime RTPCR.The omicron variant was confirmed by whole-genome sequencing performed by the China Disease Control and Prevention Agency.The demographic and clinical characteristics and laboratory test results of the patients were analysed.This study was approved by the Institutional Review Board of the Affiliated Children's Hospital of Xi'an Jiaotong University.

Clinical data collection
We collected information from the electronic medical records of paediatric patients who tested positive for SARS-CoV-2 nucleic acids.The demographic data included age, weight, and sex.All the patients were from China.The clinical data included diagnosis, clinical signs, symptoms, and treatment.Among them, convulsions, maximum body temperature, and fever duration upon admission were highlighted.The laboratory data included a full blood cell count, hypersensitive C-reactive protein (hs-CRP), liver function, and myocardial enzymes.

Statistical analysis
Statistical analysis was performed using SPSS software (version 22.0).Continuous variables were described using the mean, median, and range.Additionally, categorical variables were described as frequency rates and percentages.

Demographic and clinical data
A total of 4,520 paediatric patients with SARS-CoV-2 Omicron variant infections were included in this study.Of these, 3,861 (85.36%) were outpatients, 659 (14.64%) were hospitalised patients, and 9 (0.20%) patients died.The proportion of males was 58.14% and females was 41.86%.The age of onset ranged from newborns to 18 years of age; patients aged 1-3 years accounted for the main proportion.The median patient age was 2.08 (0-18) years (Table 1).

Characteristics of main diagnoses in patients who died
Of the nine patients who died, five were diagnosed with acute necrotising encephalopathy (ANE); two cases were associated with congenital malformations, which were hypoplastic cerebellum and intestinal duplication, and three cases were associated with bronchopneumonia.Of these patients, three were diagnosed with bronchopneumonia or severe pneumonia associated with cerebral palsy, congenital heart disease, and gangliosidosis, and one had viral encephalitis with developmental delay and pneumonia (Figure 3).

Clinical features of the patients who died (n = 9)
The main early symptoms and signs of ANE were high or very high fever, convulsions, and coma.The procalcitonin (PCT) level was obviously elevated, which is uncommon in viral infections.Of the nine patients who died, five were male and four were female.The median patient age was 1.92 (0.41-14.58) years.The main symptoms of pneumonia, were cough, cyanosis, phlegm, wet rales, and wheezing (Table 3).The blood cell analysis results were unremarkable.High-sensitivity C-reactive protein (CRP) levels were elevated in only one case of severe pneumonia.However, the procalcitonin (PCT) level was obviously elevated in six patients (mean: 41.74 ± 15.30).The pH was also markedly abnormal in six patients (mean: 7.22 ± 0.09).Blood lactic acid (LAC) was obviously elevated in five patients (mean: 4.62 ± 1.81), alanine aminotransferase (ALT) in four patients (mean: 493.78 ± 254.97), aspartate aminotransferase (AST) in seven patients (mean: 1,042.78± 582.48), and creatine kinase isoenzyme (CK-MB) in five patients (mean: 102.33 ± 26.65) (Table 4).
In brain nuclear magnetic resonance imaging (MRI), bilateral thalamic symmetrical damage were observed in all patients with ANE.The typical lesion showed restricted diffusion in the thalami and swelling and increased T2-weighted signal (Figures 4A,B   The main diagnostic composition of hospitalised patients.The abscissa is the diagnosis and accompanying symptoms.The ordinate is the frequency of the patients.PCT were significant negatively correlated with death (r s = −0.139,p = 0.001) and negatively correlated with ANE (r s = −0.081,p = 0.045).Percentage of monocytes were significant positively correlated with death (r s =0.129, p = 0.001).The correlation result show that children with SARS-CoV-2 Omicron Variant infection had higher levels of AST, ALT, CK-MB and PCT correlated with more risk of ANE and death.Elevated AST maybe was a predictors of convulsions.Elevated percentage of monocytes maybe was a protective factors of death (Figure 5).

Predictive factors for severe disease requiring hospitalization
A logistic analysis was performed.The results showed that fever time upon admission (hour), peak fever, WBC, neutrophils percentage, CRP and convulsion frequency were independent risk factors for hospitalization of children with Omicron Variant infection (P < 0.05).The odds ratio values were 1.022, 0.943, 1.072, 0.976, 1.005 and 2.844 respectively as shown in Table 5.

Discussion
Strong natural immunity is acquired after the primary infection by SARS-CoV-2 and may last for more than one year (12).However, Nguyen et al. reported that the severity of the second SARS-CoV-2 infection was similar to that of the first infection (13).Therefore, it is important to understand the pathogenic characteristics of SARS-CoV-2 in children.This was a descriptive study on the clinical characteristics of COVID-19, including data on 4,520 paediatric patients with Omicron variant infections.The mortality rate was 0.20% and the rate of hospitalization was 14.64%.
The aetiopathogenesis of SARS-CoV-2 infection in humans is reveals itself as mild symptoms to severe respiratory failure (14).In this study, 3,861 (85.36%) patients were outpatients with mild symptoms.The initial and main presenting symptom was a fever.The most common accompanying symptoms were sequent to cough (29.20%), convulsions (13.50%), vomiting (9.07%), hoarseness of voice (6.04%).The most typical symptom is a significant increase in convulsions.This indicates that nervous system damage in children with SARS-CoV-2 infection is more significant.
Of the nine patients who died, five were diagnosed with acute necrotising encephalopathy (ANE), of which the main symptoms and signs were high fever or extremely high fever, convulsions, and coma; two cases were associated with hypoplastic cerebellum and intestinal duplication; three cases were associated with bronchopneumonia; one was viral encephalitis with developmental delay and pneumonia.Six (66.67%) patients died of damage to the central nervous system.The pathogenesis is unknown.In a recent animal study, SARS-CoV-2 infections   20).It can also act as a receptor for SARS-COV-2, mediating viral entry into cerebral cells and spreading to the infective area (20).The SARS-COV-2 uses the ACE2 receptor expressed by alveolar epithelial cells to infect the host, causes lung injury, and recruits immune cells (19), which triggers a strong immune response known as cytokine storm syndrome (14).The "cytokine storm" can lead to widespread endothelial dysfunction and apoptosis in multiple organs (19).It is hypothesised that this mechanism also exists in cerebral vessels.When the cerebral vascular endothelial cells are injured, thrombus formation and occlusion can easily occur.In this study, all ANE cases showed ischaemia and necrotising changes in the basal ganglia, which is consistent with this mechanism.They often present as bilateral symmetrical lesions typically in the thalamus.The uniform and symmetrical distribution damage due to the energy depletion state (21,22).This is a dynamic process that corresponds to clinical and pathophysiological changes, such as: cerebral edema-pitting, hemorrhage, necrosisdegenerative changes (23).Some patients may experience complete regression of the lesions.But others may have residual lesions such as atrophy, white matter cyst, hypothalamic density, and hemosiderin deposition (21,24).Further studies are required to investigate the mechanisms of symmetrical damage.Among the 659 inpatients, the main diagnoses were pneumonia (36.27%),URTI (25.95%), and bronchitis (15.33%).Common diagnoses were viral encephalitis, seizure, sepsis, appendicitis, immune thrombocytopenia, nephrosis and nephritis, vomiting and diarrhoea, complicated underlying diseases, encephalopathy, leukaemia and aplastic anaemia, surgical disease, ANE, Kawasaki disease.Rare diagnoses included gastrointestinal bleeding, allergic purpura, systemic lupus erythematosus, nephritis, diabetes, cerebral infarction, cerebrospinal meningitis, fulminant myocarditis, and arrhythmia.This indicates that decease easily lead to multiple organ dysfunctions (25, 26).Febrile seizures occurred mainly in URTI (84 cases, 49.12%), bronchitis (35 cases, 34.65%), and pneumonia (11 cases, 4.60%).Myocardial damage often occurred.The clinical characteristics of the disease include respiratory infections that can invade various organs and systems.A high incidence of febrile seizures is a typical characteristic of children infected with SARS-CoV-2.However, further studies are required to elucidate the underlying pathogenic mechanisms.
Of the 3,861 outpatients, the main diagnoses were URTI (88.16%), bronchitis (9.82%), and pneumonia (2.02%).Among the 1,035 patients with URTI, the patients presented with convulsion (37.29%).Among the 32 patients with bronchitis, the patients presented with convulsions (18.75%).The clinical characteristics of outpatients were respiratory infections with various systemic injuries and a high incidence rate of febrile seizures.Upon comparison of adult patients with children with SARS-CoV-2 infections, the injuries in adults were mainly in the respiratory and cardiovascular systems, whereas the injuries in children were in the respiratory and nervous systems.The pathogenic characteristics in children are particularly similar to mouse model research (17).Although adult patients with SARS-CoV-2 infections presents a wide variety of neurological manifestations that range from mild to severe symptoms, such as anosmia, dysgeusia, myalgia, headache, hallucinations, psychomotor agitation, encephalopathy, vertigo, brain haemorrhage, brain ischaemia and encephalitis (27)(28)(29), the most common causes of death are ARDS, severe viral pneumonia, and multiple organ failure (30,31).However, differences in pathogenicity between adults and children remain unknown.The mortality rates of hospitalised patients (in our study 9/659: 1.37%) in children are much lower than in adults (4.3%) (8), and in general, the younger the age, the milder the disease presentation.
The blood cell analysis had no typical features except the percentage of monocytes was slightly elevated.Among 659 inpatients patients, ALT was elevated in 17.91% of patients; AST was elevated in 49.01% of patients, and CK-MB were elevated in 46.28% of patients.This indicates that SARS-CoV-2 infection can cause multiple organ dysfunction (25, 26).PCT levels were elevated in 25.34% patients; however, this phenomenon is rare in viral infections, and the mechanism is not known.The limitations of this study are the short period of cases and the single-center study.The long-term 2019-ncov infection effects of children are still unclear, and further in-depth studies are urgently needed.The Spearman's correlation analyses result show that children with SARS-CoV-2 Omicron Variant infection had higher levels of AST, ALT, CK-MB and PCT correlated with more risk of ANE and death.ALT, AST and CK-MB are important clinical  indicators of disease severity.Regulated by endotoxins and cytokines, PCT may also be triggered by widespread tissue damage and endothelial dysfunction (32).The logistic analyses result showed that fever time upon admission (hour), peak fever, WBC, neutrophils percentage, CRP and convulsion frequency were independent risk factors for hospitalization of children with Omicron Variant infection.There is a relative paucity of pediatric data in the management of COVID-19 (33).Our research data will be useful for future research.Summary, in the study, our findings is meaningful for further research of COVID-19 pathogenesis in children.The complex comorbidities, particularly nervous system damage are the clinical characteristic of pediatric patients with SARS-CoV-2 Omicron Variant infection.It is benefit to clinical management the severe coronavirus disease 2019 in in pediatric populations.Compare to previous pediatric COVID-19 literature, the same finding was neurologic diseases were significantly higher, with the high occurrence of seizures (34).But we analyzed the characteristics of the fatalities and showed imaging features of acute necrotizing encephalopathy.This study has several limitations.First, since this study was conducted in a limited number of big Children's hospital, there was potential selection bias.Second, due to the retrospective review of medical records, the judgment of the severity of illness was potentially misclassified and miss useful information.Finally, our data do not represent the national situation.Might prospective multicenter studies can analysis risk factors of the acute necrotizing encephalopathy, which lead main causes of death in pediatric patients with SARS-CoV-2 Omicron Variant infection.It is aim to provide a basis for early identification and treatment.

Conclusions
COVID-19 initially presents with "flu"-like symptoms.The most common symptom in children is a significant increase in the number of convulsions.If there is a high or ultra-high fever and progressive disturbance of consciousness, it is necessary to be alert to the occurrence of encephalitis, which can progress to life-threatening systemic inflammation and multi-organ dysfunction.Acute necrotising encephalopathy and pneumonia with comorbidities are the main causes of death in children with SARS-CoV-2 infection.High incidence of febrile seizures is a typical clinical characteristic.This indicates that nervous system damage in children with SARS-CoV-2 infection is more significant.It is significantly different from that in adults who die of respiratory distress syndrome (white lungs).
: red arrows).Symmetrical, multifocal thalamic damage is distinctive of ANE.It always involves the brainstem and cerebellum (Figure 4A: black arrow), and the parietal cortex (Figure 4A: yellow arrow), periventricular white matter, and the corpus callosum (Figure 4B: yellow arrow), and basal ganglia (Figure 4B: white arrow), and external and internal capsule injuries (Figure 4A: white arrow), and the centrum semiovale (Figure 4B: black arrow), temporal lobe, and amygdala.All patients had bilaterally symmetrical lesions, often showing as cytotoxic oedema, ischaemia, and necrotising changes in the lesion area.

FIGURE 1
FIGURE 1 Characteristics of main diagnoses and concomitant symptoms in outpatients.(A) Constituent ratio of main diagnoses in outpatients; (B) upper respiratory tract infection (URTI) with concomitant symptoms.(C) Bronchitis with concomitant symptoms.(D) Pneumonia with concomitant symptoms.

FIGURE 3
FIGURE 3The main diagnosis of the 9 patients who died.ANE: acute necrotizing encephalopathy.

FIGURE 4
FIGURE 4 Brain nuclear magnetic resonance (MRI) of acute necrotizing encephalopathy.(A) Bilateral thalamic symmetrical damage in T2-weighted and T2weighted flair (red arrow).Brain stem and cerebellum injured in T2-weighted (black arrow).The cerebral cortex was extensively oedematous with significant damage to the left parietal cortex (yellow arrow) in T2-weighted flair.External and internal capsule injury in T2-weighted flair (white arrow).(B) Bilateral thalamic symmetrical damage in T2-weighted flair (red arrow).Periventricular white matter damage in T2-weighted flair (yellow arrow).Centrum semiovale damaged in T2-weighted flair (black arrow).Basal ganglia damaged in T2-weighted flair and T1-weighted (white arrow).

TABLE 1
Demographics and clinical characteristics of 4,520 patients infected with the omicron variant.

TABLE 2
Laboratory characteristics of paediatric patients infected with the omicron variant.

TABLE 3
Clinical characteristics of 9 patients infected with the omicron variant who died.

TABLE 4
Blood analysis of the 9 patients infected with the omicron variant who died.

TABLE 5
Logistic analysis for the related factors predicting hospitalization of children with omicron variant infection.